Experimental validation of a parallel hybrid modular multilevel voltage source converter for HVDC transmission

This paper discusses a small scale laboratory model constructed to validate the operation of a recently proposed parallel hybrid modular multilevel voltage source converter which is intended for HVDC applications. The converter is characterized by a `chainlink´ arrangement composed of a number of half bridge cells capable of operating with bidirectional current and an associated full bridge converter for each converter phase arrangement. Each half bridge cell is made up of two bidirectional switches and a local storage capacitor. The voltages on the local storage capacitors of the `chainlink´ cells need to be controlled within a tolerance band to ensure sustainable operation of the converter. The paper also presents a capacitor voltage balancing scheme for this converter and further demonstrates black start capability using a lower rated auxiliary DC source. Four quadrant operation of the converter is also demonstrated as would be expected of a grid connected voltage source converter. Simulation and experimental results are presented to support the converter operation.